Transmit-receive FM-CW radar apparatus

ABSTRACT

A transmit-receive FM-CW radar apparatus according to one mode of the invention comprises: a mixer for downconverting an IF signal; a switch provided on the input side of the mixer; and a switch controller for controlling the switch on and off in different modes and selecting the IF signal in the different modes for supply to said mixer. A transmit-receive FM-CW radar apparatus according to another mode of the invention comprises: a mixer for downconverting an IF signal; a switch for turning on and off a local signal to be supplied to the mixer; and a switch controller for controlling the switch on and off in different modes and selecting the local signal in the different modes for supply to the mixer.

[0001] Applicant claims the right to priority from, and incorporates byreference the entire disclosure of Japanese Patent Application No.2003-78246, filed Mar. 20, 2003.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an FM-CW radar apparatus thatuses a frequency-modulated (FM) continuous wave (CW) transmit signaland, more particularly, to a transmit-receive FM-CW radar apparatus thatswitches between transmission and reception by time division.

[0004] 2. Description of the Related Art

[0005] FM-CW radar is used as a radar system for measuring the distance,the relative velocity, etc. of a target object. As FM-CW radar canmeasure the distance and the relative velocity of a vehicle travelingahead by using a simple signal processing circuit, and as itstransmitter and receiver can be constructed with simple circuitry, thistype of radar is used as an automotive collision avoidance radar.

[0006] There is disclosed as a transmit-receive FM radar a time-divisionmultiplexing FM radar system that does not require the provision of ahigh-frequency, high-gain receiving amplifier circuit which suppliesreflected FM frequencies, received via a transmit-receive commonantenna, to a common mixer while amplifying the received frequenciesintermittently in time division fashion (refer to Japanese UnexaminedPatent Publication No. H10-90397).

[0007] There is also disclosed an FM-CW radar apparatus that subtractsFM-AM conversion noise from a beat signal thereby removing the FM-AMconversion noise before the beat signal is input to an A/D converter(refer to Japanese Unexamined Patent Publication No. 2002-189074).

[0008] There is further disclosed a transmit-receive FM-CW radarapparatus that can reduce the leakage of noise components betweentransmitter and receiver (refer to Japanese Unexamined PatentPublication No. 11-148972).

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to reduce FM-AM conversionnoise in a transmit-receive FM-CW radar apparatus.

[0010] A transmit-receive FM-CW radar apparatus which switches betweentransmission and reception by time division control according to thepresent invention comprises: a mixer for downconverting an IF signal; aswitch provided on an input side of the mixer; and a switch controllerfor controlling the switch on and off in different modes and selectingthe IF signal in the different modes for supply to the mixer.

[0011] In one preferred mode of the invention, the radar apparatuscomprises a plurality of mixers, each for downconverting the IF signal,and a plurality of switches one each provided on the input side of eachof the plurality of mixers, and the switch controller controls theplurality of switches on and off in different modes and selects the IFsignal in the different modes for supply to the plurality of mixersrespectively (FIG. 7).

[0012] In another preferred mode of the invention, the radar apparatuscomprises: a selector switch for supplying the IF signal to each of theplurality of mixers by switching a connection thereof between themixers; and a switching controller for controlling timing for connectingthe selector switch to each of the plurality of mixers, and for causingthe selector switch to select the IF signal in the different modes forsupply to each of the plurality of mixers (FIG. 12).

[0013] According to the transmit-receive FM-CW radar apparatus of thepresent invention, the mixer for downconverting the IF signal is asingle mixer, and the radar apparatus includes: a switch for turning onand off the IF signal to be input to the mixer; and a mode selector forcontrolling the switch on and off in different modes while selecting theon/off mode by switching between the different modes (FIG. 14).

[0014] According to the transmit-receive FM-CW radar apparatus of thepresent invention, the mixer for downconverting the IF signal is asingle mixer, and the switch for turning on and off the IF signal to beinput to the mixer is provided on the input side of the mixer, whereinthe radar apparatus includes a mode controller for turning the switch onand off in a specific mode (FIG. 16).

[0015] The different modes consist of a short-range mode for selectingan IF signal containing a signal from a short-range target, a mid-rangemode for selecting an IF signal containing a signal from a mid-rangetarget, and a long-range mode for selecting an IF signal containing asignal from a long-range target (FIGS. 9 and 10).

[0016] The mode selector switches the mode to any one of the differentmodes, i.e., the short-range mode, the mid-range mode, or the long-rangemode. Alternatively, the mode selector switches the mode sequentiallythrough the short-range mode, the mid-range mode, and the long-rangemode (FIGS. 9 and 10).

[0017] The specific mode is any one of the above modes, i.e., theshort-range mode, the mid-range mode, or the long-range mode (FIGS. 9and 10).

[0018] The different modes consist of a mode for selecting an IF signalcorresponding to a portion occupying up to a point about {fraction(1/3)} from the leading edge of a received reflected wave, a mode forselecting an IF signal corresponding to a portion occupying up to apoint about {fraction (2/3)} from the leading edge of the receivedreflected wave, and a mode for selecting an IF signal corresponding toan entire portion of the received reflected wave (FIG. 9).

[0019] Alternatively, the different modes consist of a mode forselecting an IF signal corresponding to a portion occupying up to apoint about {fraction (1/3)} from the leading edge of a receivedreflected wave, a mode for selecting an IF signal corresponding to aportion occupying from the point about {fraction (1/3)} to the pointabout {fraction (2/3)} from the leading edge of the received reflectedwave, and a mode for selecting an IF signal corresponding to a portionoccupying from the point about {fraction (2/3)} to the point about{fraction (3/3)} from the leading edge of the received reflected wave(FIG. 10).

[0020] The mode selector switches the mode to any one of the differentmodes, i.e., the mode for selecting the IF signal corresponding to theportion occupying up to the point about {fraction (1/3)} from theleading edge of the received reflected wave, the mode for selecting theIF signal corresponding to the portion occupying up to the point about{fraction (2/3)} from the leading edge of the received reflected wave,or the mode for selecting the IF signal corresponding to the entireportion of the received reflected wave. Alternatively, the mode selectorswitches the mode sequentially through the above modes (FIG. 9).

[0021] The mode selector switches the mode to any one of the differentmodes, i.e., the mode for selecting the IF signal corresponding to theportion occupying up to the point about {fraction (1/3)} from theleading edge of the received reflected wave, the mode for selecting theIF signal corresponding to the portion occupying from the point about{fraction (1/3)} to the point about {fraction (2/3)} from the leadingedge of the received reflected wave, or the mode for selecting the IFsignal corresponding to the portion occupying from the point about{fraction (2/3)} to the point about {fraction (3/3)} from the leadingedge of the received reflected wave. Alternatively, the mode selectorswitches the mode sequentially through the above modes (FIG. 10).

[0022] The specific mode is any one of the modes consisting of the modefor selecting the IF signal corresponding to the portion occupying up tothe point about {fraction (1/3)} from the leading edge of the receivedreflected wave, the mode for selecting the IF signal corresponding tothe portion occupying up to the point about {fraction (2/3)} from theleading edge of the received reflected wave, and the mode for selectingthe IF signal corresponding to the entire portion of the receivedreflected wave (FIG. 9).

[0023] Alternatively, the specific mode is any one of the modesconsisting of the mode for selecting the IF signal corresponding to theportion occupying up to the point about {fraction (1/3)} from theleading edge of the received reflected wave, the mode for selecting theIF signal corresponding to the portion occupying from the point about{fraction (1/3)} to the point about {fraction (2/3)} from the leadingedge of the received reflected wave, and the mode for selecting an IFsignal corresponding to a portion occupying from the point about{fraction (2/3)} to the point about {fraction (3/3)} from the leadingedge of the received reflected wave (FIG. 10).

[0024] A transmit-receive FM-CW radar apparatus which switches betweentransmission and reception by time division control according to thepresent invention comprises: a mixer for downconverting an IF signal; aswitch for turning on and off a local signal to be supplied to each of aplurality of mixers; and a switch controller for controlling the switchon and off in different modes and selecting the local signal in thedifferent modes for supply to the mixer.

[0025] In one preferred mode of the invention, the radar apparatuscomprises a plurality of mixers, each for downconverting the IF signal,and a plurality of switches one each provided for each of the pluralityof mixers, and the switch controller controls the plurality of switchesin different modes and selects the local signal in the different modesfor supply to the plurality of mixers respectively (FIG. 11).

[0026] According to the transmit-receive FM-CW radar apparatus of thepresent invention, the mixer for downconverting the IF signal is asingle mixer, and the switch for turning on and off the local signal isprovided for the single mixer, wherein the radar apparatus includes amode selector for controlling the switch on and off in different modeswhile selecting the on/off mode by switching between the different modes(FIG. 15).

[0027] According to the transmit-receive FM-CW radar apparatus of thepresent invention, the mixer for downconverting the IF signal is asingle mixer, and the switch for turning on and off the local signal isprovided for the single mixer, wherein the radar apparatus includes amode controller for turning the switch on and off in a specific mode(FIG. 17).

[0028] The different modes consist of a short-range mode fordownconverting an IF signal containing a signal from a short-rangetarget, a mid-range mode for downconverting an IF signal containing asignal from a mid-range target, and a long-range mode for downconvertingan IF signal containing a signal from a long-range target (FIGS. 9 and10).

[0029] The mode selector switches the mode to any one of the differentmodes, i.e., the short-range mode, the mid-range mode, or the long-rangemode. Alternatively, the mode selector switches the mode sequentiallythrough the short-range mode, the mid-range mode, and the long-rangemode (FIGS. 9 and 10).

[0030] The specific mode is any one of the modes consisting of the modesequentially through the short-range mode, the mid-range mode, and thelong-range mode (FIGS. 9 and 10).

[0031] The different modes consist of a mode for downconverting an IFsignal corresponding to a portion occupying up to a point about{fraction (1/3)} from the leading edge of a received reflected wave, amode for downconverting an IF signal corresponding to a portionoccupying up to a point about {fraction (2/3)} from the leading edge ofthe received reflected wave, and a mode for downconverting an IF signalcorresponding to an entire portion of the received reflected wave (FIG.9).

[0032] Alternatively, the different modes consist of a mode fordownconverting an IF signal corresponding to a portion occupying up to apoint about {fraction (1/3)} from the leading edge of a receivedreflected wave, a mode for downconverting an IF signal corresponding toa portion occupying from the point about {fraction (1/3)} to the pointabout {fraction (2/3)} from the leading edge of the received reflectedwave, and a mode for downconverting an IF signal corresponding to aportion occupying from the point about {fraction (2/3)} to the pointabout {fraction (3/3)} from the leading edge of the received reflectedwave (FIG. 10).

[0033] The mode selector switches the mode to any one of the differentmodes, i.e., the mode for downconverting the IF signal corresponding tothe portion occupying up to the point about {fraction (1/3)} from theleading edge of the received reflected wave, the mode for downconvertingthe IF signal corresponding to the portion occupying up to the pointabout {fraction (2/3)} from the leading edge of the received reflectedwave, or the mode for downconverting the IF signal corresponding to theentire portion of the received reflected wave. Alternatively, the modeselector switches the mode sequentially through the above modes (FIG.9).

[0034] The mode selector switches the mode to any one of the differentmodes, i.e., the mode for downconverting the IF signal corresponding tothe portion occupying up to the point about {fraction (1/3)} from theleading edge of the received reflected wave, the mode for downconvertingthe IF signal corresponding to the portion occupying from the pointabout {fraction (1/3)} to the point about {fraction (2/3)} from theleading edge of the received reflected wave, or the mode fordownconverting the IF signal corresponding to the portion occupying fromthe point about {fraction (2/3)} to the point about {fraction (3/3)}from the leading edge of the received reflected wave. Alternatively, themode selector switches the mode sequentially through the above modes(FIG. 10).

[0035] The specific mode is any one of the modes consisting of the modefor downconverting the IF signal corresponding to the portion occupyingup to the point about {fraction (1/3)} from the leading edge of thereceived reflected wave, the mode for downconverting the IF signalcorresponding to the portion occupying up to the point about {fraction(2/3)} from the leading edge of the received reflected wave, and themode for downconverting the IF signal corresponding to the entireportion of the received reflected wave (FIG. 9).

[0036] Alternatively, the specific mode is any one of the modesconsisting of the mode for downconverting the IF signal corresponding tothe portion occupying up to the point about {fraction (1/3)} from theleading edge of the received reflected wave, the mode for downconvertingthe IF signal corresponding to the portion occupying from the pointabout {fraction (1/3)} to the point about {fraction (2/3)} from theleading edge of the received reflected wave, and the mode fordownconverting the IF signal corresponding to the portion occupying fromthe point about {fraction (2/3)} to the point about {fraction (3/3)}from the leading edge of the received reflected wave (FIG. 10).

[0037] In the transmit-receive FM-CW radar according to the presentinvention, since the signals are processed separately according to thetarget range, such as the short range, mid range, and long range, FM-AMconversion noise can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above object and features of the present invention will bemore apparent from the following description of the preferredembodiments with reference to the accompanying drawings, wherein:

[0039]FIGS. 1A, 1B, and 1C are diagrams for explaining the principle ofFM-CW radar when the relative velocity with respect to target is 0;

[0040]FIGS. 2A, 2B, and 2C are diagrams for explaining the principle ofFM-CW radar when the relative velocity with respect to target is v;

[0041]FIG. 3 is a diagram showing one configuration example of asingle-antenna transmit-receive FM-CW radar;

[0042]FIGS. 4A, 4B, 4C, and 4D are diagrams showing timings fortransmission, reception, etc.;

[0043]FIG. 5 is a diagram showing a portion of the configuration of thesingle-antenna transmit-receive FM-CW radar shown in FIG. 3;

[0044]FIGS. 6A, 6B, 6C, 6D, and 6E are diagrams for explaining therelationships of a modulating signal VT to an output frequency f andpower P from a modulating signal generator and an output voltage Vd froma mixer;

[0045]FIG. 7 is a diagram showing the configuration of a firstembodiment of the present invention;

[0046]FIGS. 8A, 8B, 8C, and 8D are diagrams showing where in a reflectedwave the signals from short-range, mid-range, and long-range targets,respectively, are contained;

[0047]FIGS. 9A, 9B, 9C, and 9D are diagrams showing the on/off operationof switches S1 to S3 for receiving the signals from the short-range,mid-range, and long-range targets, respectively;

[0048]FIGS. 10A, 10B, 10C, and 10D are diagrams showing the on/offoperation of the switches S1 to S3 for receiving the signals from theshort-range, mid-range, and long-range targets, respectively;

[0049]FIG. 11 is a diagram showing the configuration of a secondembodiment of the present invention;

[0050]FIG. 12 is a diagram showing the configuration of a thirdembodiment of the present invention;

[0051]FIGS. 13A and 13B are diagrams for explaining the switching timingof a switch S shown in FIG. 12;

[0052]FIG. 14 is a diagram showing the configuration of a fourthembodiment of the present invention;

[0053]FIG. 15 is a diagram showing the configuration of a fifthembodiment of the present invention;

[0054]FIG. 16 is a diagram showing the configuration of a sixthembodiment of the present invention;

[0055]FIG. 17 is a diagram showing the configuration of a seventhembodiment of the present invention;

[0056]FIGS. 18A, 18B, and 18C are diagrams showing filtercharacteristics according to an eighth embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] Before describing the radar apparatus of the present invention,the principle of FM-CW radar will be described.

[0058] An FM-CW radar measures the distance to a target object, such asa vehicle traveling ahead, by transmitting a continuous wavefrequency-modulated, for example, in a triangular pattern. Morespecifically, the transmitted wave from the radar is reflected by thevehicle ahead, and the reflected signal is received and mixed with thetransmitted signal to produce a beat signal (radar signal). This beatsignal is fast Fourier transformed to analyze the frequency. Thefrequency-analyzed beat signal exhibits a peak at which power becomeslarge in correspondence with the target. The frequency corresponding tothis peak is called the peak frequency. The peak frequency carriesinformation about distance, and the peak frequency differs between therising portion and falling portion of the triangular FM-CW wave due tothe Doppler effect associated with the relative velocity with respect tothe vehicle ahead. The distance and the relative velocity with respectto the vehicle ahead can be obtained from the peak frequencies in therising and falling portions. If there is more than one vehicle travelingahead, a pair of peak frequencies in the rising and falling portions isgenerated for each vehicle. Forming pairs of peak frequencies in therising and falling portions is called pairing.

[0059]FIGS. 1A to 1C are diagrams for explaining the principle of theFM-CW radar when the relative velocity with respect to the target is 0.The transmitted wave is a triangular wave whose frequency changes asshown by a solid line in FIG. 1A. In the figure, f0 is the centerfrequency of the transmitted wave, Δf is the FM modulation width, and Tmis the repetition period. The transmitted wave is reflected from thetarget and received by an antenna; the received wave is shown by adashed line in FIG. 1A. The round trip time T to and from the target isgiven by T=2r/C, where r is the distance (range) to the target and C isthe velocity of radio wave propagation.

[0060] Here, the received wave is shifted in frequency from thetransmitted signal (i.e., produces a beat) according to the distancebetween the radar and the target.

[0061] The frequency component fb of the beat signal can be expressed bythe following equation.

fb=fr=(4·Δf/C·Tm)r  (1)

[0062] where fr is the frequency due to the range (distance).

[0063]FIGS. 2A to 2C, on the other hand, are diagrams for explaining theprinciple of the FM-CW radar when the relative velocity with respect tothe target is v. The frequency of the transmitted wave changes as shownby a solid line in FIG. 2A. The transmitted wave is reflected from thetarget and received by the antenna; the received wave is shown by adashed line in FIG. 2A. Here, the received wave is shifted in frequencyfrom the transmitted signal (i.e., produces a beat) according to thedistance between the radar and the target.

[0064] In this case, since the relative velocity with respect to thetarget is v, a Doppler shift occurs, and the beat frequency component fbcan be expressed by the following equation.

fb=fr±fd=(4·Δf/C·Tm)r±(2·f 0 /C)v  (2)

[0065] where fr is the frequency due to the distance, and fd is thefrequency due to the velocity.

[0066] The symbols in the above equation have the following meanings.

[0067] fb: Transmit beat frequency

[0068] fr: Range (distance) frequency

[0069] fd: Velocity frequency

[0070] f0: Center frequency of transmitted wave

[0071] Δf: Frequency modulation width

[0072] Tm: Period of modulation wave

[0073] C: Velocity of light (velocity of radio wave)

[0074] T: Round trip time of radio wave to and from target object

[0075] r: Range (distance) to target object

[0076] v: Relative velocity with respect to target object

[0077]FIG. 3 is a diagram showing one configuration example of asingle-antenna transmit-receive FM-CW radar. As shown, a modulatingsignal generator (MOD) 1 applies a modulating signal to avoltage-controlled oscillator (VCO) 2 for frequency modulation, and thefrequency-modulated wave is passed through a directional coupler 3 andtransmitted out from a transmitting/receiving antenna (ATR), while aportion of the transmitted signal is separated by the directionalcoupler 3 and fed into a first mixer 4-1. The signal reflected from atarget is received by the transmitting/receiving antenna (ART). SW8 is atransmit-receive switch which switches the antenna between transmissionand reception in accordance with a signal supplied from atransmit-receive switching signal generator (OSC) 9 constructed from anoscillator. The OSC 9 generates a modulating signal of frequency f_(sw)for causing the SW 8 to switch the antenna between transmission andreception. The received signal is mixed in the first mixer 4-1 with theoutput signal of the voltage-controlled oscillator (VCO) 2 to produce anIF signal. The IF signal is mixed in a second mixer 4-2 with themodulating signal of frequency f_(sw) supplied from the OSC 9 and isthus downconverted, producing a beat signal. The beat signal is passedthrough a filter (F) 5, and is converted by an A/D converter (A/D) 6into a digital signal; the digital signal is then supplied to a digitalsignal processor (DSP) 7 where signal processing such as fast Fouriertransform is applied to the digital signal to obtain distance, relativevelocity, etc.

[0078] The power of the received signal received via thetransmitting/receiving antenna and the power of the beat signal are asshown below. First, the power of the received signal, Pr, is expressedby the following equation.

Pr={(G ²·λ² ·σ·Pt)/((4π)³ ·r ⁴)}·La  (3)

[0079] The symbols in the above equation have the following meanings.

[0080] G: Antenna gain

[0081] λ: Wavelength

[0082] σ: Cross-sectional area of reflecting object

[0083] Pt: Transmitter power

[0084] r: Range

[0085] La: Atmospheric attenuation factor

[0086] The power of the beat signal, Pb, is expressed by the followingequation.

Pb=Pr ·Cmix  (4)

[0087] where Cmix is the conversion loss factor in the mixer.

[0088]FIGS. 4A to 4D are diagrams showing timings for transmission,reception, etc. The SW 8 in FIG. 3 is switched by the signal offrequency f_(sw) (period T_(sw)) to switch the timing betweentransmission and reception. FIG. 4A shows the transmit timing interval,and FIG. 4B shows the return timing of the transmitted wave reflectedfrom a target. FIG. 4C shows the receive timing interval; the reflectedwave returned during this interval is received by the antenna ATR andfed into the mixer. Accordingly, when the reflected wave is returned atthe timing shown in FIG. 4B, the actually received reflected wave is asshown in FIG. 4D.

[0089] As described above, in the single-antenna transmit-receive FM-CWradar, the transmit and receive timings are provided one alternatingwith the other, and the reflected wave, i.e., the transmitted wavereturned upon reflection from the target, is received. Further, sincethe receive timing interval is one half the cycle period T_(sw) of thetransmit-receive switching frequency, the receiving efficiency ismaximized when the delay time of the reflected wave is one half thecycle period; on the other hand, if the delay time is one cycle period,the reflected wave cannot be received.

[0090] Accordingly, to secure the desired detection range, thetransmit-receive switching frequency must be selected so that the delaytime of the reflected wave returned from the desired detection rangewill be less than one cycle period of the transmit-receive switchingfrequency. In this case, if a target at longer range is also to bedetected, a lower transmit-receive switching frequency is selected.

[0091] On the other hand, in the single-antenna transmit-receive FM-CWradar, noise occurs during FM-AM conversion, and this degrades the S/Nratio. The principle of why noise occurs during the conversion will bedescribed below with reference to FIG. 5 and FIGS. 6A to 6E.

[0092]FIG. 5 is a diagram showing a portion of the configuration of thesingle-antenna transmit-receive FM-CW radar shown in FIG. 3. As shown inFIG. 5, the modulating signal generator (MOD) 1 applies a modulatingsignal VT to the voltage-controlled oscillator (VCO) 2 for frequencymodulation. A transmitted signal of frequency f and output power P isoutput from the VCO 2, and a portion αP (α<1) of the transmitted signalis separated by the directional coupler 3 and fed into the first mixer4-1. On the other hand, the received signal is mixed in the first mixer4-1 with the output signal of the voltage-controlled oscillator (VCO) 2to produce the IF signal. The IF signal is mixed in the second mixer 4-2with the modulating signal of frequency f_(sw) supplied from the OSC 9and is thus downconverted, producing a beat signal of voltage Vd.

[0093]FIGS. 6A to 6E are diagrams for explaining the relationships ofthe modulating signal VT to the output frequency f and power P from theVCO 2 and the voltage Vd of the beat signal output from the second mixer4-2. FIG. 6A is a graph showing the relationship between VT and f. As VTchanges from Va to Vb and to Vc, f changes from fa to fb and to fc.Here, even when VT changes, P should not change but remain constant, butactually, P also changes as shown in FIG. 6B.

[0094] As for Vd, if P remained constant irrespective of the change ofVT, Vd would also remain constant, but as it is, Vd also changes asshown in FIG. 6C, because P changes. As a result, when the voltage VTapplied to the VCO 2 changes as shown in FIG. 6D, the voltage Vd of thebeat signal output from the second mixer 4-2 also changes as shown inFIG. 6E. This change causes FM-AM conversion noise, which is introducedinto the mixer output, resulting in a degradation of the S/N ratio.

[0095] The present invention aims to reduce the FM-AM conversion noiseand to improve the S/N ratio. Embodiments of the present invention willbe described below.

[0096] [Embodiment 1]

[0097]FIG. 7 is a diagram showing the configuration of atransmit-receive FM-CW radar according to a first embodiment of thepresent invention. The configuration of this embodiment differs from theconfiguration shown in FIG. 3 by the inclusion of a plurality ofswitches S1 to S3 and a corresponding plurality of second mixers 4-2(1)to 4-2(3), filters 5-1 to 5-3, and A/D converters 6-1 to 6-3. In thisembodiment, the switches S1 to S3 are provided with switch controllersCtr1 to Ctr3, respectively, and are controlled on and off inrespectively different modes. Accordingly, the IF signal output from thefirst mixer 4-1 is selected in the respectively different modes, and theIF signal thus selected is supplied to the corresponding one of theplurality of second mixers 4-2(1) to 4-2(3), where the IF signal ismixed with the modulating signal from the OSC 9 and is thusdownconverted, producing a beat signal. The produced beat signal isindividually processed in the corresponding one of the filters 5-1 to5-3 and the corresponding one of the A/D converters 6-1 to 6-3.

[0098] The shorter the target range, the earlier the reflected wavereturns. FIGS. 8A to 8D are diagrams showing which portion of thereflected wave is received according to the target range.

[0099]FIG. 8D is a diagram showing the receive timing interval (the sameas that shown in FIG. 4C), and FIG. 8A is a diagram showing the returntiming of the reflected wave from a short-range target. As can be seenfrom the waveform shown in FIG. 8A, the reflected wave from theshort-range target returns during the interval t_(a) to t₁ which isearlier than the receive timing interval t₀ to t₃. Here, since theportion t_(a) to t₀ of the reflected wave returns earlier than thereceive timing interval t₀ to t₃, this portion is not received and, ofthe reflected wave from the short-range target, only the portion t₀ tot₁ is actually received.

[0100] Likewise, FIG. 8B is a diagram showing the return timing of thereflected wave from a mid-range target. In this case, as can be seenfrom the waveform shown in FIG. 8B, since the reflected wave returnsduring the interval t_(b) to t₂ which is earlier than the receive timinginterval, only the portion t₀ to t₂ is actually received.

[0101]FIG. 8C is a diagram showing the return timing of the reflectedwave from a long-range target. In this case, since the reflected wavereturns during the interval that substantially coincides with thereceive timing interval, most of the reflected wave is received.

[0102] Here, the short range refers to a distance of about 50 m or lessand the mid range to a distance about 50 m to 100 m, while the longrange refers to a distance longer than about 100 m. However, these areonly examples, and the ranges need not necessarily be limited to thesedistances.

[0103] The signal from the short-range target is contained in theportion of the reflected wave shown in FIG. 8A, the signal from themid-range target is contained in the portion of the reflected wave shownin FIG. 8B, and the signal from the long-range target is contained inthe portion of the reflected wave shown in FIG. 8C.

[0104] In view of this, in the present invention, the switches S1 to S3are turned on and off in respectively different modes to select the IFsignal in the respectively different modes, and the signals from theshort-range, mid-range, and long-range targets are supplied to therespective mixers 4-2(1) to 4-2(3) and processed separately from eachother, thereby reducing FM-AM reconversion noise and thus improving theS/N ratio.

[0105]FIGS. 9A to 9D are diagrams showing the on/off operation of S1 toS3. Since S1 selects the IF signal containing the signal from theshort-range target, S1 is turned on only for the duration of theinterval t₀ to t₁ so as to select the IF signal corresponding to theportion occupying up to the point about {fraction (1/3)} from theleading edge of the received reflected wave, and supplies the IF signalonly for that portion to the second mixer 4-2(1) (short-range mode).Since S2 selects the IF signal containing the signal from the mid-rangetarget, S2 is turned on only for the duration of the interval t₀ to t₂so as to select the IF signal corresponding to the portion occupying upto the point about {fraction (2/3)} from the leading edge of thereceived reflected wave, and supplies the IF signal only for thatportion to the second mixer 4-2(2) (mid-range mode). Since S3 selectsthe IF signal containing the signal from the long-range target, S3 isturned on only for the duration of the interval t₀ to t₃ so as to selectthe IF signal corresponding to the entire portion of the receivedreflected wave, and supplies the IF signal only for that portion to thesecond mixer 4-2(3) (long-range mode). Here, the interval t₀ to t₃coincides with the receive timing interval shown in FIG. 9D (refer toFIG. 4C for the receive timing interval).

[0106] The switch controllers Ctr1 to Ctr3 perform the on/off control ofthe respective switches S1 to S3 in the respectively different modes,based on the signal of frequency f_(sw) supplied from the OSC 9.

[0107] As described above, since the signal contained in the reflectedwave is selectively supplied according to the target range, the FM-AMconversion noise can be reduced and the S/N ratio improved, comparedwith the case where all the reflected wave incident during the receivetiming interval is supplied.

[0108] The above description has dealt with the case where threeswitches are provided, but the number of switches may be variedaccording to the range. For example, two switches, one for theshort-range mode and the other for the long-range mode, may be provided,or the modes from the short-range to the long-range may be divided intofour or more modes. Further, the reflected wave to be selected has beendivided into three portions, but this is just one example; the onlyrequirement here is that the reflected wave be divided so that thesignals from the short-range, mid-range, and long-range targets, forexample, can be individually selected.

[0109] As shown in FIG. 7, the signal separated by the directionalcoupler 3 for application to the first mixer 4-1 is being output at alltimes irrespective of the receive timing interval. However, in thepresent invention, only the selected signal is supplied to thecorresponding second mixer 4-2 for processing and, as shown in FIG. 9C,the signal is selected for the duration of the interval t₀ to t₃ at thelongest; since the transmitted signal from the coupler, which containsFM-AM conversion noise, is not supplied for the duration of the intervalT₀, the noise is reduced correspondingly.

[0110]FIGS. 10A to 10D are diagrams showing a modified example of theON/OFF operation of S1 to S3 shown in FIGS. 9A to 9D. Since S1 selectsthe IF signal containing the signal from the short-range target, S1 isturned on only for the duration of the interval t₀ to t₁ (FIG. 10A) soas to select the IF signal corresponding to the portion occupying up tothe point about {fraction (1/3)} from the leading edge of the receivedreflected wave, and supplies the IF signal only for that portion to thesecond mixer 4-2(1) (short-range mode). Since S2 selects the IF signalcontaining the signal from the mid-range target, S2 is turned on onlyfor the duration of the interval t₁ to t₂ (FIG. 10B) so as to select theIF signal corresponding to the portion occupying from the point about{fraction (1/3)} to the point about {fraction (2/3)} from the leadingedge of the received reflected wave, and supplies the IF signal only forthat portion to the second mixer 4-2(2) (mid-range mode). Since S3selects the IF signal containing the signal from the long-range target,S3 is turned on only for the duration of the interval t₂ to t₃ (FIG.10C) so as to select the IF signal corresponding to the portionoccupying from the point about {fraction (2/3)} to the point about{fraction (3/3)} from the leading edge of the received reflected wave,and supplies the IF signal only for that portion to the second mixer4-2(3) (long-range mode). FIG. 10D is a diagram showing the receivetiming interval.

[0111] [Embodiment 2]

[0112]FIG. 11 is a diagram showing the configuration of atransmit-receive FM-CW radar according to a second embodiment of thepresent invention. The configuration is the same as that shown in FIG. 7in that the plurality of second mixers 4-2(1) to 4-2(3), filters 5-1 to5-3, and A/D converters 6-1 to 6-3 are provided. However, thisembodiment differs in that the plurality of second mixers 4-2(1) to4-2(3) downconvert the IF signal by using respectively different localsignals and thus select the respective range signal components to beextracted from the IF signal, so that the signals from the short-range,mid-range, and long-range targets are separately processed in the DSP.

[0113] S1 to S3 are connected between the OSC 9 and the respectivesecond mixers 4-2(1) to 4-2(3), and are controlled on and off inrespectively different modes by the respective controllers Ctr1 to Ctr3.When S1 to S3 are respectively turned on, the respective second mixers4-2(1) to 4-2(3) downconvert the IF signal. Since S1 to S3 arecontrolled on and off in respectively different modes by the respectivecontrollers Ctr1 to Ctr3, the respective second mixers 4-2(1) to 4-2(3)downconvert the IF signal by using the local signals of different modes.The controllers Ctr1 to Ctr3 control the on/off operations of therespective switches S1 to S3 based on the signal of frequency f_(sw)supplied from the OSC 9.

[0114] The on/off timings of S1 to S3 in FIG. 11 are the same as thoseshown in FIGS. 9A to 9C. As shown in FIG. 9A, for the mixer 4-2(1) todownconvert the IF signal containing the signal from the short-rangetarget, S1 is turned on only for the duration of the interval t₀ to t₁(FIG. 9A) so as to generate a local signal with a duty ratiocorresponding to the portion occupying up to the point about {fraction(1/3)} from the leading edge of the receiving interval (short-rangemode). For the mixer 4-2(2) to downconvert the IF signal containing thesignal from the mid-range target, S2 is turned on only for the durationof the interval t₀ to t₂ (FIG. 9B) so as to generate a local signal witha duty ratio corresponding to the portion occupying up to the pointabout {fraction (2/3)} from the leading edge of the receiving interval(mid-range mode). For the mixer 4-2(3) to downconvert the IF signalcontaining the signal from the long-range target, S3 is turned on forthe duration of the receive timing interval t₀ to t₃ (FIG. 9C) so as togenerate a local signal with a duty ratio corresponding to the entireportion of the receiving interval (long-range mode).

[0115] The on/off timings of S1 to S3 in FIG. 11 may be made the same asthose shown in FIGS. 10A to 10C. As shown in FIG. 10A, for the mixer4-2(1) to downconvert the IF signal containing the signal from theshort-range target, S1 is turned on only for the duration of theinterval t₀ to t₁ so as to generate a local signal with a duty ratio andphase corresponding to the portion occupying up to the point about{fraction (1/3)} from the leading edge of the receiving interval(short-range mode). For the mixer 4-2(2) to downconvert the IF signalcontaining the signal from the mid-range target, S2 is turned on onlyfor the duration of the interval t₁ to t₂ (FIG. 10B) so as to generate alocal signal with a duty ratio and phase corresponding to the portionoccupying from the point about {fraction (1/3)} to the point about{fraction (2/3)} from the leading edge of the receiving interval(mid-range mode). For the mixer 4-2(3) to downconvert the IF signalcontaining the signal from the long-range target, S3 is turned on forthe duration of the interval t₂ to t₃ (FIG. 10C) so as to generate alocal signal with a duty ratio and phase corresponding to the portionoccupying from the point about {fraction (2/3)} to the point about{fraction (3/3)} from the leading edge of the receiving interval(long-range mode). FIG. 10D shows the receive timing interval.

[0116] The duty ratios shown in FIGS. 10A to 10D are all identical, butdiffer only in phase.

[0117] [Embodiment 3]

[0118]FIG. 12 is a diagram showing the configuration of atransmit-receive FM-CW radar according to a third embodiment of thepresent invention. The configuration is the same as that shown in FIG. 7in that the plurality of second mixers 4-2(1) to 4-2(3), filters 5-1 to5-3, and A/D converters 6-1 to 6-3 are provided. In this embodiment, aselector switch S is provided which is switched for connection to one ofthe plurality of second mixers 4-2(1) to 4-2(3). Here, by controllingthe timing with which the selector switch is connected to the respectivesecond mixers 4-2(1) to 4-2(3), the IF signal is selected inrespectively different modes and supplied to the respective secondmixers 4-2(1) to 4-2(3).

[0119]FIGS. 13A and 13B are diagrams for explaining the switching timingof the switch S shown in FIG. 12. The switching of the selector switch Sis controlled by a switching controller (SWc) 10, that is, the SWc 10controls the switching timing of the switch S based on the modulatingsignal of frequency f_(sw) supplied from the OSC 9.

[0120]FIG. 13B shows the receive timing interval, while FIG. 13A showsthe switching timing of the switch S. The switch S connects to the mixer4-2(1) only for the duration of the interval t₀ to t₁, the portionoccupying up to the point about {fraction (1/3)} from the leading edgeof the receive timing interval, so that the IF signal containing thesignal from the short-range target is selected and downconverted. Next,the switch S connects to the mixer 4-2(2) only for the duration of theinterval t₁ to t₂, the portion occupying from the point about {fraction(1/3)} to the point about {fraction (2/3)} from the leading edge of thereceive timing interval, so that the IF signal containing the signalfrom the mid-range target is selected and downconverted. Finally, theswitch S connects to the mixer 4-2(3) only for the duration of theinterval t₂ to t₃, the portion occupying from the point about {fraction(2/3)} to the point about {fraction (3/3)} from the leading edge of thereceive timing interval, so that the IF signal containing the signalfrom the long-range target is selected and downconverted.

[0121] [Embodiment 4]

[0122]FIG. 14 is a diagram showing the configuration of atransmit-receive FM-CW radar according to a fourth embodiment of thepresent invention. In this embodiment, a mode selector (MDsw) 11 isprovided, and the on/off operation of the switch S is controlled inaccordance with the mode selected by the switching of the MDsw 11.

[0123] In this embodiment, the on/off timing of the switch S is variedin accordance with the mode selected by the switching of the MDsw 11,for example, the short-range mode, the mid-range mode, or the long-rangemode.

[0124] The on/off timings in the respective modes are the same as thoseshown in FIGS. 9A to 9C. The MDsw 11 performs the control based on themodulating signal of frequency f_(sw) supplied from the OSC 9.

[0125] The mode switching can be performed based on the target range.For example, if the target is at short range, the mode is switched tothe short-range mode. Alternatively, the mode may be switched cyclicallythrough the short-range mode, the mid-range mode, and the long-rangemode in this order.

[0126] The on/off operation of the switch S in the respective mode maybe performed in accordance with the on/off timings shown in FIGS. 10A to10C.

[0127] [Embodiment 5]

[0128]FIG. 15 is a diagram showing the configuration of atransmit-receive FM-CW radar according to a fifth embodiment of thepresent invention. In this embodiment, a mode selector (MDsw) 11 isprovided, and the on/off operation of the switch S is controlled inaccordance with the mode selected by the switching of the MDsw 11,thereby controlling the local signal with which the second mixer 4-2downconverts the IF signal.

[0129] In this embodiment, the local signal with which the second mixer4-2 downconverts the IF signal is controlled by turning the switch S onand off based on the mode selected by the switching of the MDsw 11,thereby selecting the range signal to be extracted from the IF signalwhich is to be downconverted, and the signals from the short-range,mid-range, and long-range targets are processed separately.

[0130] The mode switching can be performed based on the target range.For example, if the target is at short range, the mode is switched tothe short-range mode. Alternatively, the mode may be switched cyclicallythrough the short-range mode, the mid-range mode, and the long-rangemode in this order.

[0131] The MDsw 11 controls the switch S based on the signal offrequency f_(sw) supplied from the OSC 9.

[0132] The on/off timings of the switch S are the same as those employedin the fourth embodiment. That is, the on/off operation of the switch Sis performed in accordance with the on/off timings shown in FIGS. 9A to9C or FIGS. 10A to 10C.

[0133] [Embodiment 6]

[0134]FIG. 16 is a diagram showing the configuration of atransmit-receive FM-CW radar according to a sixth embodiment of thepresent invention. This embodiment is a modification of the fourthembodiment shown in FIG. 14; that is, the mode selector 11 is replacedby a mode controller (MD) 12 operable in a specific mode, and the on/offoperation of the switch S is controlled based on the specific mode.

[0135] For example, when the MD 12 for the specific mode is set as thecontroller for the short-range mode, the on/off timing of the switch Sis the same as that shown in FIG. 9A or FIG. 10A. When the MD 12 is setas the controller for the mid-range mode, the switch S is turned on andoff with the timing shown in FIG. 9B or FIG. 10B, while when the MD 12is set as the controller for the long-range mode, the switch S is turnedon and off with the timing shown in FIG. 9C or FIG. 10C.

[0136] The MD 12 controls the switch S in the specific mode based on thesignal of frequency f_(sw) supplied from the OSC 9.

[0137] [Embodiment 7]

[0138]FIG. 17 is a diagram showing the configuration of atransmit-receive FM-CW radar according to a seventh embodiment of thepresent invention. This embodiment is a modification of the fifthembodiment shown in FIG. 15; that is, the mode selector 11 is replacedby a mode controller (MD) 12 operable in a specific mode, and the on/offoperation of the switch S is controlled based on the specific mode.

[0139] For example, when the MD 12 for the specific mode is set as thecontroller for the short-range mode, the on/off timing of the switch Sis the same as that shown in FIG. 9A or FIG. 10A. When the MD 12 is setas the controller for the mid-range mode, the switch S is turned on andoff with the timing shown in FIG. 9B or FIG. 10B, while when the MD 12is set as the controller for the long-range mode, the switch S is turnedon and off with the timing shown in FIG. 9C or FIG. 10C.

[0140] The MD 12 controls the switch S in the specific mode based on thesignal of frequency f_(sw) supplied from the OSC 9.

[0141] [Embodiment 8]

[0142]FIGS. 18A to 18C are diagrams showing filter characteristics in atransmit-receive FM-CW radar according to an eighth embodiment of thepresent invention. In the eighth embodiment, the characteristics of thefilters provided for the respective mixers in the transmit-receive FM-CWradars shown in FIGS. 7, 11, and 12 are varied in accordance with therespective modes, thereby efficiently reducing the FM-AM conversionnoise.

[0143]FIG. 18A shows the characteristic of the filter for the beatsignal containing the signal from the short-range target, FIG. 18B showsthe characteristic of the filter for the beat signal containing thesignal from the mid-range target, and FIG. 18C shows the characteristicof the filter for the beat signal containing the signal from thelong-range target. Much of the noise is contained in the beat signalcontaining the signal from the short-range target. Accordingly, as shownin FIGS. 18B and 18C, the filter for the mid range and the filter forthe long range are each constructed to have a characteristic that cutsoff the low-frequency components of the beat signal, thus cutting offthe signal from the short-range target to remove the noise containedtherein.

What is claimed is:
 1. A transmit-receive FM-CW radar apparatus whichswitches between transmission and reception by time division control,comprising: a mixer for downconverting an IF signal; a switch providedon an input side of said mixer; and a switch controller for controllingsaid switch on and off in different modes and selecting said IF signalin said different modes for supply to said mixer.
 2. A transmit-receiveFM-CW radar apparatus as claimed in claim 1, wherein said radarapparatus comprises a plurality of said mixers, each for downconvertingsaid IF signal, and a plurality of said switches one each provided onthe input side of each of said plurality of mixers, and wherein saidswitch controller controls said plurality of switches on and off indifferent modes and selects said IF signal in said different modes forsupply to said plurality of mixers respectively.
 3. A transmit-receiveFM-CW radar apparatus as claimed in claim 1, wherein said radarapparatus comprises: a plurality of said mixers, each for downconvertingsaid IF signal; a selector switch for supplying said IF signal to eachof said plurality of mixers by switching a connection thereof betweensaid mixers; and a switching controller for controlling timing forconnecting said selector switch to each of said plurality of mixers, andfor causing said selector switch to select said IF signal in saiddifferent modes for supply to each of said plurality of mixers.
 4. Atransmit-receive FM-CW radar apparatus as claimed in claim 1, whereinsaid mixer is a single mixer, and said radar apparatus includes: aswitch, provided on the input side of said single mixer, for turning onand off said IF signal to be input to said mixer; and a mode selectorfor controlling said switch on and off in different modes whileselecting said on/off mode by switching between said different modes. 5.A transmit-receive FM-CW radar apparatus as claimed in claim 1, whereinsaid mixer is a single mixer, and said radar apparatus includes: aswitch, provided on the input side of said single mixer, for turning onand off said IF signal to be input to said mixer; and a mode controllerfor turning said switch on and off in a specific mode.
 6. Atransmit-receive FM-CW radar apparatus as claimed in any one of claims 1to 3, wherein said different modes consist of a short-range mode forselecting an IF signal containing a signal from a short-range target, amid-range mode for selecting an IF signal containing a signal from amid-range target, and a long-range mode for selecting an IF signalcontaining a signal from a long-range target.
 7. A transmit-receiveFM-CW radar apparatus as claimed in claim 4, wherein said mode selectorswitches said mode to any one of said different modes which consist of ashort-range mode for selecting an IF signal containing a signal from ashort-range target, a mid-range mode for selecting an IF signalcontaining a signal from a mid-range target, and a long-range mode forselecting an IF signal containing a signal from a long-range target. 8.A transmit-receive FM-CW radar apparatus as claimed in claim 4, whereinsaid mode selector switches said mode sequentially through a short-rangemode for selecting an IF signal containing a signal from a short-rangetarget, a mid-range mode for selecting an IF signal containing a signalfrom a mid-range target, and a long-range mode for selecting an IFsignal containing a signal from a long-range target.
 9. Atransmit-receive FM-CW radar apparatus as claimed in claim 5, whereinsaid specific mode is any one of modes consisting of a short-range modefor selecting an IF signal containing a signal from a short-rangetarget, a mid-range mode for selecting an IF signal containing a signalfrom a mid-range target, and a long-range mode for selecting an IFsignal containing a signal from a long-range target.
 10. Atransmit-receive FM-CW radar apparatus as claimed in any one of claims 1to 3, wherein said different modes consist of a mode for selecting an IFsignal corresponding to a portion occupying up to a point about{fraction (1/3)} from a leading edge of a received reflected wave, amode for selecting an IF signal corresponding to a portion occupying upto a point about {fraction (2/3)} from the leading edge of said receivedreflected wave, and a mode for selecting an IF signal corresponding toan entire portion of said received reflected wave.
 11. Atransmit-receive FM-CW radar apparatus as claimed in any one of claims 1to 3, wherein said different modes consist of a mode for selecting an IFsignal corresponding to a portion occupying up to a point about{fraction (1/3)} from a leading edge of a received reflected wave, amode for selecting an IF signal corresponding to a portion occupyingfrom the point about {fraction (1/3)} to a point about {fraction (2/3)}from the leading edge of said received reflected wave, and a mode forselecting an IF signal corresponding to a portion occupying from thepoint about {fraction (2/3)} to a point about {fraction (3/3)} from theleading edge of said received reflected wave.
 12. A transmit-receiveFM-CW radar apparatus as claimed in claim 4, wherein said mode selectorswitches said mode to any one of said different modes which consist of amode for selecting an IF signal corresponding to a portion occupying upto a point about {fraction (1/3)} from a leading edge of a receivedreflected wave, a mode for selecting an IF signal corresponding to aportion occupying up to a point about {fraction (2/3)} from the leadingedge of said received reflected wave, and a mode for selecting an IFsignal corresponding to an entire portion of said received reflectedwave.
 13. A transmit-receive FM-CW radar apparatus as claimed in claim4, wherein said mode selector switches said mode sequentially through amode for selecting an IF signal corresponding to a portion occupying upto a point about {fraction (1/3)} from a leading edge of a receivedreflected wave, a mode for selecting an IF signal corresponding to aportion occupying up to a point about {fraction (2/3)} from the leadingedge of said received reflected wave, and a mode for selecting an IFsignal corresponding to an entire portion of said received reflectedwave.
 14. A transmit-receive FM-CW radar apparatus as claimed in claim4, wherein said mode selector switches said mode to any one of saiddifferent modes which consist of a mode for selecting an IF signalcorresponding to a portion occupying up to a point about {fraction(1/3)} from a leading edge of a received reflected wave, a mode forselecting an IF signal corresponding to a portion occupying from thepoint about {fraction (1/3)} to a point about {fraction (2/3)} from theleading edge of said received reflected wave, and a mode for selectingan IF signal corresponding to a portion occupying from the point about{fraction (2/3)} to a point about {fraction (3/3)} from the leading edgeof said received reflected wave.
 15. A transmit-receive FM-CW radarapparatus as claimed in claim 4, wherein said mode selector switchessaid mode sequentially through a mode for selecting an IF signalcorresponding to a portion occupying up to a point about {fraction(1/3)} from a leading edge of a received reflected wave, a mode forselecting an IF signal corresponding to a portion occupying from thepoint about {fraction (1/3)} to a point about {fraction (2/3)} from theleading edge of said received reflected wave, and a mode for selectingan IF signal corresponding to a portion occupying from the point about{fraction (2/3)} to a point about {fraction (3/3)} from the leading edgeof said received reflected wave.
 16. A transmit-receive FM-CW radarapparatus as claimed in claim 5, wherein said specific mode is any oneof modes consisting of a mode for selecting an IF signal correspondingto a portion occupying up to a point about {fraction (1/3)} from aleading edge of a received reflected wave, a mode for selecting an IFsignal corresponding to a portion occupying up to a point about{fraction (2/3)} from the leading edge of said received reflected wave,and a mode for selecting an IF signal corresponding to an entire portionof said received reflected wave.
 17. A transmit-receive FM-CW radarapparatus as claimed in claim 5, wherein said specific mode is any oneof modes consisting of a mode for selecting an IF signal correspondingto a portion occupying up to a point about {fraction (1/3)} from aleading edge of a received reflected wave, a mode for selecting an IFsignal corresponding to a portion occupying from the point about{fraction (1/3)} to a point about {fraction (2/3)} from the leading edgeof said received reflected wave, and a mode for selecting an IF signalcorresponding to a portion occupying from the point about {fraction(2/3)} to a point about {fraction (3/3)} from the leading edge of saidreceived reflected wave.
 18. A transmit-receive FM-CW radar apparatuscomprising: a mixer for downconverting an IF signal; a switch forturning on and off a local signal to be supplied to said mixer; and aswitch controller for controlling said switch on and off in differentmodes and selecting said local signal in said different modes for supplyto said mixer.
 19. A transmit-receive FM-CW radar apparatus as claimedin claim 18, wherein said radar apparatus comprises a plurality of saidmixers, each for downconverting said IF signal, and a plurality of saidswitches one each provided for each of said plurality of mixers, andwherein said switch controller controls said plurality of switches indifferent modes and selects said local signal in said different modesfor supply to said plurality of mixers respectively.
 20. Atransmit-receive FM-CW radar apparatus as claimed in claim 18, whereinsaid mixer is a single mixer, and said switch for turning on and offsaid local signal is provided for said single mixer, and wherein saidradar apparatus includes a mode selector for controlling said switch onand off in different modes while selecting said on/off mode by switchingbetween said different modes.
 21. A transmit-receive FM-CW radarapparatus as claimed in claim 18, wherein said mixer is a single mixer,and said switch for turning on and off said local signal is provided forsaid single mixer, and wherein said radar apparatus includes a modecontroller for turning said switch on and off in a specific mode.
 22. Atransmit-receive FM-CW radar apparatus as claimed in any one of claims18 or 19, wherein said different modes consist of a short-range mode fordownconverting an IF signal containing a signal from a short-rangetarget, a mid-range mode for downconverting an IF signal containing asignal from a mid-range target, and a long-range mode for downconvertingan IF signal containing a signal from a long-range target.
 23. Atransmit-receive FM-CW radar apparatus as claimed in claim 20, whereinsaid mode selector switches said mode to any one of said different modeswhich consist of a short-range mode for downconverting an IF signalcontaining a signal from a short-range target, a mid-range mode fordownconverting an IF signal containing a signal from a mid-range target,and a long-range mode for downconverting an IF signal containing asignal from a long-range target.
 24. A transmit-receive FM-CW radarapparatus as claimed in claim 20, wherein said mode selector switchessaid mode sequentially through a short-range mode for downconverting anIF signal containing a signal from a short-range target, a mid-rangemode for downconverting an IF signal containing a signal from amid-range target, and a long-range mode for downconverting an IF signalcontaining a signal from a long-range target.
 25. A transmit-receiveFM-CW radar apparatus as claimed in claim 21, wherein said specific modeis any one of modes consisting of a short-range mode for downconvertingan IF signal containing a signal from a short-range target, a mid-rangemode for downconverting an IF signal containing a signal from amid-range target, and a long-range mode for downconverting an IF signalcontaining a signal from a long-range target.
 26. A transmit-receiveFM-CW radar apparatus as claimed in any one of claims 18 or 19, whereinsaid different modes consist of a mode for downconverting an IF signalcorresponding to a portion occupying up to a point about {fraction(1/3)} from a leading edge of a received reflected wave, a mode fordownconverting an IF signal corresponding to a portion occupying up to apoint about {fraction (2/3)} from the leading edge of said receivedreflected wave, and a mode for downconverting an IF signal correspondingto an entire portion of said received reflected wave.
 27. Atransmit-receive FM-CW radar apparatus as claimed in any one of claims18 or 19, wherein said different modes consist of a mode fordownconverting an IF signal corresponding to a portion occupying up to apoint about {fraction (1/3)} from a leading edge of a received reflectedwave, a mode for downconverting an IF signal corresponding to a portionoccupying from the point about {fraction (1/3)} to a point about{fraction (2/3)} from the leading edge of said received reflected wave,and a mode for downconverting an IF signal corresponding to a portionoccupying from the point about {fraction (2/3)} to a point about{fraction (3/3)} from the leading edge of said received reflected wave.28. A transmit-receive FM-CW radar apparatus as claimed in claim 20,wherein said mode selector switches said mode to any one of saiddifferent modes which consist of a mode for downconverting an IF signalcorresponding to a portion occupying up to a point about {fraction(1/3)} from a leading edge of a received reflected wave, a mode fordownconverting an IF signal corresponding to a portion occupying up to apoint about {fraction (2/3)} from the leading edge of said receivedreflected wave, and a mode for downconverting an IF signal correspondingto an entire portion of said received reflected wave.
 29. Atransmit-receive FM-CW radar apparatus as claimed in claim 20, whereinsaid mode selector switches said mode sequentially through a mode fordownconverting an IF signal corresponding to a portion occupying up to apoint about {fraction (1/3)} from a leading edge of a received reflectedwave, a mode for downconverting an IF signal corresponding to a portionoccupying up to a point about {fraction (2/3)} from the leading edge ofsaid received reflected wave, and a mode for downconverting an IF signalcorresponding to an entire portion of said received reflected wave. 30.A transmit-receive FM-CW radar apparatus as claimed in claim 20, whereinsaid mode selector switches said mode to any one of said different modeswhich consist of a mode for downconverting an IF signal corresponding toa portion occupying up to a point about {fraction (1/3)} from a leadingedge of a received reflected wave, a mode for downconverting an IFsignal corresponding to a portion occupying from the point about{fraction (1/3)} to a point about {fraction (2/3)} from the leading edgeof said received reflected wave, and a mode for downconverting an IFsignal corresponding to a portion occupying from the point about{fraction (2/3)} to a point about {fraction (3/3)} from the leading edgeof said received reflected wave.
 31. A transmit-receive FM-CW radarapparatus as claimed in claim 20, wherein said mode selector switchessaid mode sequentially through a mode for downconverting an IF signalcorresponding to a portion occupying up to a point about {fraction(1/3)} from a leading edge of a received reflected wave, a mode fordownconverting an IF signal corresponding to a portion occupying fromthe point about {fraction (1/3)} to a point about {fraction (2/3)} fromthe leading edge of said received reflected wave, and a mode fordownconverting an IF signal corresponding to a portion occupying fromthe point about {fraction (2/3)} to a point about {fraction (3/3)} fromthe leading edge of said received reflected wave.
 32. A transmit-receiveFM-CW radar apparatus as claimed in claim 21, wherein said specific modeis any one of modes consisting of a mode for downconverting an IF signalcorresponding to a portion occupying up to a point about {fraction(1/3)} from a leading edge of a received reflected wave, a mode fordownconverting an IF signal corresponding to a portion occupying up to apoint about {fraction (2/3)} from the leading edge of said receivedreflected wave, and a mode for downconverting an IF signal correspondingto an entire portion of said received reflected wave.
 33. Atransmit-receive FM-CW radar apparatus as claimed in claim 21, whereinsaid specific mode is any one of modes consisting of a mode fordownconverting an IF signal corresponding to a portion occupying up to apoint about {fraction (1/3)} from a leading edge of a received reflectedwave, a mode for downconverting an IF signal corresponding to a portionoccupying from the point about {fraction (1/3)} to a point about{fraction (2/3)} from the leading edge of said received reflected wave,and a mode for downconverting an IF signal corresponding to a portionoccupying from the point about ⅔ to a point about {fraction (3/3)} fromthe leading edge of said received reflected wave.